LIDAR

TRANSCRIPT

NARRATIONFrom their window, astronauts see these ghostly tendrils at the very edge of space, the last place you’d expect to find clouds. And from the ground, only in the high latitudes during summer, you might be lucky enough to see them too…the highest clouds on our planet, like luminous ripples on an atmospheric lake. They’re called noctilucent clouds, meaning they glow in the dark. Because they’re nearly a hundred kilometres high, they reflect the sun even after it has slipped below the horizon.

Andrew KlekociukNoctilucent clouds are special phenomena in the polar atmosphere during the summertime. The clouds were only discovered in the late 1800s. Before that they were completely unknown.

NARRATIONIf there’s one place with enough clean air and big sky views to investigate mysterious clouds of ice crystals, it’s here.

Mark HorstmanI’m at Davis station in Antarctica, and this yellow hut behind me is changing our understanding of how greenhouse gases affect the atmosphere.

NARRATIONFrom this high-tech shipping container, amid the shimmer of the southern lights, a powerful laser beams into the night sky. It’s called a LIDAR.

Andrew KlekociukLIDAR stands for light detection and ranging. It’s a technique very similar to radar except we’re using light pulses from a laser, rather than radio waves, to probe the atmosphere, sending out these pulses and listening for the echoes that are coming back.

NARRATIONWith mirrors and filters, the LIDAR directs the laser through telescopes on the roof. They can pinpoint an unexplored part of the atmosphere and use light to take its temperature.

Andrew KlekociukWe make a direct measurement of the density of the atmosphere with the laser beam. The amount of light that’s scattered back is directly proportional to density and we can use that information with a knowledge of the way the gases behave in the atmosphere to tell us something with the temperature. It’s quite a remarkable measurement.

Mark HorstmanWhat are you testing there, Andrew?

Andrew KlekociukI’m having a look at the shape of the laser beam to make sure everything is aligned inside the laser carefully – it’s ready to go. You can see the laser beam up in the sky there, Mark. That’s going up to about six kilometres in the cloud, but we can measure light coming back from a hundred kilometres away.

NARRATIONThat may not be far along the earth’s surface, but after a hundred kilometres straight up, the atmosphere runs out and space begins. More than three-quarters of our planet’s air mass, and most of its weather, is in the lowest 10 kilometres, the troposphere. It gets colder with altitude... Until it hits the second layer, the stratosphere, where the opposite happens. The higher you go, the warmer it gets, as the ozone layer here absorbs UV radiation.

Andrew KlekociukBy about 50 kilometres, the temperatures are similar to what we have near the surface. But the pressures are extremely low. The density of the atmosphere is about one ten-thousandth that at sea level.

NARRATIONAfter 50 kilometres, the next layer is the mesosphere, and the temperature starts dropping again. By 85 kilometres, the top of the mesosphere is the coldest place on earth. The density of the upper atmosphere is so low that carbon dioxide actually radiates heat away into space, rather than trapping it.

NARRATIONToo high for balloons and even aircraft, but too low for satellites, the mesosphere is the least known layer. This is the birthplace of noctilucent clouds.

Mark HorstmanAnd this is what you’ve been seeking?

Andrew KlekociukThat’s right. This one’s actually appeared for a period of about four hours. It’s incredibly cold up there during the summer time. Temperatures are about minus 150 degrees. And that enables water vapour to freeze into tiny ice crystals. The highest ones we’ve ever seen are probably up around about 89 to 90 kilometres. Not much beyond that because there’s hardly any water vapour up there and the temperatures start to increase as you go above about 90 kilometres. So it’s really in this narrow band that temperatures are low enough for these clouds to exist.

Andrew KlekociukWe’ve been studying these clouds at Davis for the last few years to try and build up a picture of whether the conditions in the upper atmosphere are changing.

NARRATIONAnd there’s more change happening than they thought. While the lower atmosphere has warmed by less than one degree over recent decades, the upper atmosphere has cooled by at least 10. At the same time, noctilucent clouds are growing brighter and seen further from the poles.

Andrew KlekociukMy main aim is to contribute information that’s going to definitively answer the questions about climate change and the atmosphere. We expect the extent and brightness of these clouds to be increasing with time. There is some evidence now from satellite measurements and from the northern hemisphere that this is happening. We’re down here in the Antarctic trying to see if we see the same trends.

NARRATIONFrom the edge of our thin spacesuit around earth, these clouds remind us that understanding climate change needs knowledge of the atmosphere from the top down, as well as the bottom up.

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Matthew Pickett - 16 Apr 2010 7:03:41am

Hi John,

Thousands of experiments undertaken around the world since the laser was invented many decades ago, have shown, (and Rayleigh and Mie scattering theory shows it too), that you can detect a backscatter signal using LIDAR. And not only a signal, but a strong signal. Granted the signal is attenuated by the atmosphere, but our technology is good enough to pick up this tiny signal. Analysis techniques have been well advanced to separate this attenuation effect from the original backscatter signal, which in turn can be related to molecular & particle densities.

In conclusion, there's no doubt that this Antarctican LIDAR is seeing noctilucent clouds at tremendous distances - it's a remarkable scientific and engineering achievement. Klekociuk and his team ought to be congratulated.

Cheers,

Matt.

John Heenan - 10 Apr 2010 3:12:10pm

Science is promoting LIDAR as a silver bullet to solve all sorts of difficult measurement problems. Rather like snake oil.

As a follow on to previous comments, it is clear I so not accept that direct scattering is responsible for light received back at the LIDAR. Rayleigh Scattering is a favourite principle mentioned in scientific papers that use LIDAR to allegedly measure temperature in the atmosphere. It is bogus science.

The idea that individual gaseous atoms will scatter light directly back is absurd. Leave aside the reasonable possibility of continuous refraction back to the LIDAR (due to continuous variations in density and density inversions). Light that may be scattered indirectly back, say as a result of Rayleigh Scattering, will only occur if the same light photon is scattered by more than one atom. In this case the results have been corrupted.

Taking a look at the equation for light intensity due to Rayleigh Scattering at http://en.wikipedia.org/wiki/Rayleigh_scattering indicates the intensity of light scattered directly back to the LIDAR device due to a single atom is zero. This is because cos(180 degrees) is -1 and makes the intensity zero from the equation.

Even if density is known it is not sufficient to measure temperature. What about pressure or partial pressure? Also the atoms that scattering is being sought from may not be in a true gaseous state. The basic science empirical equation PVT=constant only holds for pure gases. For non pure gases the equation explodes into multiple equations.

Great example of science promising more than it can deliver and the ABC being taken for a ride. Adding in the irrlevancy about noctilucent clouds takes science to new lows and laughs at our gullibility.

Here are some relevant Wikipedia entries to help add some clarity.

http://en.wikipedia.org/wiki/Rayleigh_scattering

http://en.wikipedia.org/wiki/LIDAR

http://en.wikipedia.org/wiki/Noctilucent_cloud

John Heenan

John Heenan - 10 Apr 2010 12:43:51pm

Let me be VERY SPECIFIC in the context of my last comment.

Scientist Andrew Klekociuk stated in the program "The amount of light thatâ€™s scattered back is directly proportional to density..."

This is rubbish and indicates an ignorance of basic science (as indicated in my last comment). The context makes it clear Andrew is referring to the density of the atmosphere.

The amount of laser light that is 'scattered' back has nothing to do with noctilucent clouds or with the absolute density of the atmosphere. It has everything to do with the amount of variation in density of the atmosphere (as indicated in my last comment).

It is also grossly misleading and confusing to imply noctilucent clouds reflect the light. These clouds may serve as a useful way to crudely correlate the direction of a temperature change.

John Heenan

john jarmain - 18 Apr 2010 12:41:25am

Who are you, John Heenan, what are your qualifications in this area of science, it would add weight to your comments in this matter.

John Heenan - 10 Apr 2010 11:41:49am

Typically confused, grossly over simplified and over interpreted ramblings from dogmatic scientists who don't understand the basics and who desperately want to make their results appear significant in the battle for grants and importance.

Variations in density give rise to refraction that 'bends' wave propagated energy. This is accepted basic science. Continuous variations in density can 'bend' wave energy back. This is the basis for over the horizon radar and long range radio, not the reflection or scattering dogma that every text book parrots.